1. Field of the Invention
The present invention relates to a semiconductor device, particularly to a modulation circuit. Further, the present invention relates to a semiconductor device (a data carrier) capable of wirelessly communicating (transmitting and receiving) data.
2. Description of the Related Art
In recent years, individual identification technologies have attracted attention. In these technologies, an ID number (an identification number) is assigned to each individual object, thereby revealing the information history of individual objects. These technologies are utilized for production, management, or the like. In particular, RFID (radio frequency identification) technologies used for sending and receiving data by wireless communication are spreading. A wireless communication system using RFID technologies includes a radio communication device (an interrogator) and a data carrier (a responder), and data is wirelessly transferred between them. The radio communication device refers to a device capable of wirelessly sending and receiving signals, such as a reader/writer, a cellular phone, and a personal computer; in this specification, it is referred to as a reader/writer. The data carrier is generally called an RF tag, an ID tag, an IC tag, an IC chip, a wireless tag, an electronic tag, or the like; in this specification, it is referred to as an RF tag.
An RF tag includes a variety of circuits needed in order for data to be wirelessly transferred between the RF tag and a reader/writer. One example of such a variety of circuits is a modulation circuit. A modulation circuit has a function of modulating a carrier wave outputted from a reader/writer in response to a response signal outputted from a logic circuit, and then sending a response signal to the reader/writer.
Known methods for sending a response signal to a reader/writer include a load modulation method (load switching method) in which modulation is performed by changing the load impedance between the two terminals of an antenna included in an RF tag (see Patent Document 1, for example).